Stabilization of heterocyclic compounds having a hetero nitrogen atom



United States Patent STABILIZATION OF HETEROCYCLIC COM- KIIIEIISIDSHAVING A HETERO NITROGEN Clyde W. Mertz, Borger, T ex., assignor toPhillips Petroleum Company, a corporation of Delaware No Drawing.Application January 26, 1953 Serial No. 333,391

13 Claims. (Cl. 260-290) This invention perains to the stabilization ofvinyl-sub stituted heterocyclic compounds having a hetero nitrogen atom.In one of its aspects the invention pertains to the stabilization ofvinylpyridine compounds.

It is known that difficulties are encountered in the manufacture ofvinylpyridine and other vinyl-substituted heterocyclic nitrogencompounds because of the tendency of these vinyl compounds to polymerizeupon standing at room temperature and upon exposure to elevatedtemperatures. In the case of vinylpyridines the presence of soluble andinsoluble polymers is particularly undesirable because vinylpyridinecompounds are employed as monomers for use in polymerization reactionsand the like. Hence in the preparation of vinylpyridine compounds thesteps of distillation and storage have been matters of concern.

In accordance with this invention, for the purpose of overcomingdifliculties due to the fact that vinylpyridine compounds are highlysusceptible to polymerization, it is proposed to add a novel inhibitingagent or stabilizer. I have discovered that readily polymerizablevinylpyridine compounds can be effectively stabilized againstpolymerization during distillation and storage by adding salts ofdithiocarbamic acid, such as compounds comprising the following generalformula wherein A represents an alkali-metal, an alkaline earth metal orammonium, where X represents the numerical value for the valence ofsubstituent A, where each R can be hydrogen, methyl, ethyl, propyl,butyl or amyl, or where both Rs may be included in a piperidine ringstructure, e.g., sodium pentamethylene dithiocarbamate. A preferredgroup of compounds corresponding to the above general formula is thealkali-metal salts of dialkylsubstituted dithiocarbamic acid. From thispreferred group of alkali-metal salts of dialkyl-substituteddithiocarbamic acid two compounds have been selected and have ben foundto be effective stabilizing agents, which are sodiumdimethyldithiocarbamate and potassium dirnethyldithiocarbamate and areknown throughout the synthetic rubber industry under trade names asfollows: Sodium dimethyldithiocarbamate is known as Goodrite 3954 and ismanufactured by the B. F. Goodrich Chemical Company. Potassiumdimethyldithiocarbamate is known by the trade names Olme manufactured bythe U. S. Rubber Company and Polystop K manufactured by the General Tireand Rubber Company. This invention therefore, pertains to a method fortreating polymerizable vinylpyridine compounds with salts ofdithiocarbamic acid to stabilize the pyridine compounds at distillationtemperatures as well as at room temperature and below. The process ofthis invention however is also applicable Patented Oct. 6, 1959 to othervinyl-substituted heterocyclic compounds having a hetero nitrogen atom.

The polymerization which is inhibited by the novel stabilizing agentsdisclosed herein is primarily that which forms an insoluble, hard,porous opaque material often referred to as popcorn polymer. However,polymers are also formed which are soluble in the vinylpyridinereactants and which become apparent by an increase in viscosity. As willbe seen from the examples, I have found that soluble polymer formationis also inhibited by the disclosed stabilizing agents, but to a lesserextent than is popcorn polymer formation inhibited.

One group of polymerizable vinyl-substituted heterocyclic compoundscontaining a hetero nitrogen atom which can be stabilized in accordancewith my invention is the monoand divinylpyridines, with the vinyl groupbeing present in any of the several positions in the pyridine nucleus.Alkyl groups can be present on the ring, or on the alpha carbon atom ofthe vinyl group, but the number of carbon atoms in the combined alkylgroups should generally not be greater than 12. These alkyl groups arepreferably methyl and ethyl groups. Substituents attached to carbonatoms in the ring can be selected from the group consisting of hydrogen,alkyl, vinyl, and not more than two of said groups being vinyl oralphamethylvinyl; and the total number of carbon atoms in the alkylgroups being not greater than 12. Examples of such compounds are2-vinylpyridine; 3-vinylpyridine; 4-vinylpyridine; 2,5-divinylpyridine;2-n1ethyl-5-vinylpyridine; 2- vinyl-S-ethylpyridine;2,3,4trimethyl-5-vinylpyridine; 3,4, 5,6 tetramethyl-Z-vinylpyridine;3-ethyl-5-vinylpyridine; 2,6-diethyl-4-vinylpyridine; 2isopropyl-4-nonyl-5-vinylpyridine; Z-mezhyl-S-undecyl-3-vinylpyridine;3-dodecyl- 4,5-divinylpyridine;2,4-dimethyl-5,6-dipentyl-3-vinylpyridine; 2-decyl-5-(alpha-methylvinyl)pyridine; 3,5-di(alphamethylvinyl) pyridine; and the like.

Other polymerizable heterocyclic compounds containing a hetero nitrogenatom included within the scope of this invention are those vinyl andalpha-methylvinyl substituted heterocyclic nitrogen compounds in whichthe ring structure is unsaturated, partially saturated and completelysaturated. Examples include vinyl and alpha methylvinyl substitutedquinolines, isoquinolines, piperidines (hexahydropyridines), pyrroles,pyrrolidines, pyrrolidones, alkyl derivatives of the foregoingcompounds, dihydro and tetrahydropyridines, partially hydrogenatedquinolines and isoquinolines, and pyrrolines (dihydropyrroles). Examplesof such compounds are 2-vinylquinoline; 8-ethyl-2-vinylquinoline;4-hexyl5-vinylquino line; l-vinylisoquinoline;S-methyl-l-isopropenylisoquinoline; 1,8-divinylisoquinoline;vinylpyrrolidone; vinylpyrrole; vinylpiperidine; and vinylpylrolidineand the like. Normally the vinyl substituent will be attached to a ringcarbon atom. However, in compounds wherein the ring nitrogen atom is asecondary nitrogen atom, the vinyl group can instead be attached to thisring nitrogen atom, for example, N-vinylcarbazole andN-vinylpyrrolidone. The process of this invention is particularlyapplicable to organic or hydrocarbon mixtures containing a substantialproportion of a vinylpyridine compound, i.e. that amount which causesditliculties due to its polymerization, e.g. over 50 percent. Examplesof vinylpyridine compounds contained in such mixtures are2-vinyl-4,6-dimethylpyridine, 2-methyl4-vinylpyridine, Z-methyl-S-vinylpyridine, 5-ethyl-2-vinylpyridine, S-amyl-Z-vinylpyridine.

In carrying out the invention solution of the stabilizing agent in thevinylpyridine or other heterocyclic compound containing a heteronitrogen atom to be stabilized can be accomplished in any suitablemanner, such as by agitating or stirring the mixture containing thestabilizing agent, or merely by allowing the mixture to stand.Ultimately, the decision on what concentration of the stabilizing agentto use will depend on variable factors, such as the temperature andduration of time at a given temperature at which the heterocycliccompound containing a hetero nitrogen atom will be maintained wheninhibition of polymer formation is desired, and economic considerations.In practice a range of concentration of from 0.01 percent to 5.0 percentby weight of the sodium dimethyldithiocarbarnate (Goodrite 3954") orpotassium dimethyldithiocarbamate (Olme or Polystop K) will be employed,all percentages being based on the weight of the heterocyclic compoundcontaining a hetero nitrogen atom to be stabilized. Furthermore, theamount will depend somewhat on which particular heterocyclic compoundcontaining a hetero nitrogen atom is concerned, and, as will be apparentfrom the following examples, upon the particular salt of dithiocarbamicacid being utilized.

EXAMPLE I Polymerization of 2-methyl-S-vinylpyridine at 190 F.

Sample Description Polymer Formation Observed M lxture almost solid withsome popcorn polymer in 8 hours; all popcorn polymer in less than 21hours.

Mixture very fluid and no popcorn polymer in hours; mixture a littleviscous but no popcorn polymer in 88 hours.

Mixture very fluid and no popcorn polymer In 65 hours; no popcornpolymer in 88 hours and mixture less viscous than in (b) above.

(a) Inoculated vinylpyridlne mixture.

(b1 Inoculated vinylpyridino mixture plus 0.6 weight percent ()lrnr."

(c) Inoculated vinylpyridlne mixture plus 1.0 weight percent Olme.

In addition to the above it has been found that a quantity as small as0.05 percent by weight of Goodritc 3954 was extremely effective toinhibit popcorn polymer formation in Z-methyl-S-vinylpyridine maintainedat 190 F. containing a seed of popcorn polymer and a drop of cumenehydroperoxide for up to 72 hours.

From the foregoing it is apparent that salts of dithiocarbamic acid areeffective inhibitors of popcorn, or insoluble polymer, formation whenheterocyclic compounds having a hetero nitrogen atom are maintained atelevated temperatures, and most remarkably, even when the test solutionsare seeded to make the test conditions more severe. Also, it can be seenthat these stabilizing agents retard the formation of soluble polymerssince sample (c) was less viscous after 88 hours than was sample (b)after 88 hours. The presence of large amounts of soluble polymer isindicated by an increase in viscosity. Smaller amounts can be found bydilution with hexane which yields a white precipitate when solublepolymers are present.

This invention accordingly concerns a method for treating polymerizablevinylpyridine compounds with salts of dithiocarbamic acid attemperatures up to 400 F. as well as at room temperature or below. Inaccordance with this invention organic mixtures containingvinylpyridine, i.e., vinylpyridine mixed with other organic heterocyclicor pyridine compounds, or purified vinylpyridines, can be stored overlong periods of time preferabiy at room temperature. Likewise, when itis desired to separate a particular vinylpyridine compound from otherorganic compounds, the stabilized solution can be distilled, preferablyat reduced pressure, without any substantial loss of product, thestabilizing agent serving to inhibit polymerization of the vinylpyridinecompounds during the process. The instant invention is particularlyapplicable to mixtures of heterocyclic compounds having a heteronitrogen atom, containing a substantial proportion of at least onevinyl-substituted heterocyclic compound having a hetero nitrogen atom,i.e., sufiicient to cause difiiculty due to polymer formation duringstorage and distillation.

In the practice of this invention the polymer formation inhibitingagents disclosed herein can be used, for example, in the followingmanner. In the preparation of Z-rnethyl-S-vinylpyridine fromZ-methyl-S-ethylpyridine by dehydrogenation, the principal separation isa combined steam and vacuum distillation to make a separation between2-methyl-5-ethylpyridine and 2-methy1-5- vinylpyridine. Because this isa difficult separation, due to the proximity of the boiling points ofthe stated components, considerable polymerization of the 2-methyl-5-vinylpyridine occurs. After a short period of time operation of thecolumn becomes impossible because of an insoluble polymer which forms inthe lower part of the fractionating column and in the kettle. However,by the use of salts of dithiocarbamic acid in accordance with thisinvention shut-downs due to the formation of insoluble polymer arelargely eliminated. I have found that when the object is merely tostabilize the distillate by the inhibition of insoluble polymerformation the disclosed inhibiting agents can be employed in the lowestproportion which will desirably inhibit the formation of insolublepolymer. For such purpose the feed to the distillation column can betreated with from 0.01 percent to 1.0 percent by weight of theinhibiting agent, based on the weight of the vinyl-substitutedheterocyclic compound having a hetero nitrogen atom. In many instances,when a concentration of the disclosed inhibiting agents used is greaterthan 1.0 percent by weight based on the weight of the vinyl-substitutedheterocyclic compound having a hetero nitrogen atom, i.e., from 1.0 to5.0 percent by weight, the formation of soluble polymer is inhibitedduring distillation as well as the formation of insoluble polymer. It isto be noted that the inhibiting agents disclosed herein can be addeddirectly to the reflux of the distillation column rather than to thefeed mixture in an amount sufiicient to stabilize the vinylpyridinesdepending on operating conditions then being utilized, and in many casesit will be found that a smaller quantity of inhibiting agent is requiredto produce a stabilized product when this method is followed than whenthe inhibiting agents are added to the feed mixture.

In practicing the invention, for example in the separation ofZ-methyl-S-vinylpyridine from Z-methyl-S-ethylpyridine and lightermaterial, the mixture can be distilled in a combined steam-vacuum unitoperated with a top tower pressure of millimeters mercury absolute.Sufficient steam is added to the unit to maintain a temperature of F. inthe reboiler. The Z-methyl-S- vinylpyridine is withdrawn at the bottomof the unit and Z-methyl-S-ethylpyridine of sufficient purity forrecycle purposes is obtained as an overhead product. In this columninsoluble polymer formation has been particularly troublesome prior tothe use of this invention.

It will be understood that the foregoing disclosure is illustrative andthat other embodiments within the scope of the invention will occur tothose skilled in the art. Those skilled in the art will appreciate, forinstance, that for the punposes of this invention, vinyl-substitutedheterocyclic compounds having a hetero nitrogen atom further substitutedwith non-interfering groups, for example, halo-, nitro-, amino-,hydroxyl-, and carboxyl-, are the same as nonsubstituted vinylheterocyclic compounds having a hetero nitrogen atom.

I claim:

1. A method for inhibiting the polymerization of 2-methyl-S-vinylpyridine against popcorn polymer formation which comprisestreating said Z-methyl-S-vinylpyridine with an amount suflicient toefiect said inhibition of sodium dimethyl-dithiocarbamate.

2. A method for inhibiting the polymerization of 2-methyl-S-vinylpyridine against popcorn polymer formation which comprisestreating said 2-methyl-5-vinylpyridine with an amount suflicient toeflfect said inhibition of potassium dimethyl-dithiooarbamate.

3. A method for inhibiting the polymerization of 2-methyl-S-vinylpyridine against popcorn polymer formation which comprisestreating said Z-methyl-S-vinylpyridine with from 0.01 percent to 5.0percent by weigh based on the 2-methyl-5-vinylpyridine, of sodiumdimethyl-dithiocarbamate.

4. A method for inhibiting the polymerization of 2-methyl-S-vinylpyridine against popcorn polymer formation which comprisestreating said Lmethyl-S-vinylpyridine with from 0.01 percent to 5.0percent by weight, based on the Z-methyl-S-vinylpyridine, of potassiumdimethyl-dithiocarbamate.

5. A method of stabilizing a heterocyclic nitrogen base selected fromthe group consisting of compounds having the formulae R M-s i I N(c=oH,),.

wherein n is an integer selected from the group consisting of 1 and 2, Ris selected from the group consisting of H, CH and C 11 and each R isindividually selected from the group consisting of H and an alkyl group,not more than 12 carbon atoms being present in the total of said Rgroups, against popcorn polymer formation, which comprises treating saidheterocyclic nitrogen base with a compound selected from the groupconsisting of compounds according to the general formula wherein M is amember selected from the group consisting of alkali-metals, alkalineearth metals, and ammonium, x is an integer selected from the groupconsisting of 1 and 2, and R and R are selected from the groupconsisting of H, methyl, ethyl, propyl, butyl, amyl, and methyleneradicals which, when joined together, form with the nitrogen atom, apiperidine ring.

6. The method of claim 5 wherein the heterocyclic nitrogen base istreated with about 0.01 to about 5.0 weight percent, based on saidheterocyclic nitrogen base, of sodium dimethyl-dithiocarbamate.

7. The method of claim 5 wherein the heterocyclic nitrogen base istreated with about 0.01 to about 5.0 weight percent, based on saidheterocyclic nitrogen base, of potassium dimethyl-dithiocarbamate.

8. A polymerizable hetenocyclic base selected from the group consistingof compounds having the general formulae wherein n is an integerselected from the group consisting of 1 and 2, R is selected from thegroup consisting of H, CH and C H and each R is individually selectedfrom the group consisting of H and an alkyl group, not more than 12carbon atoms being present in the total of said R groups, saidheterocyclic base being stabilized against popcorn polymer formation, bythe addition of a stabilizing amount of a compound selected from thegroup consisting of compounds according to the general formula wherein Mis a member selected from the group consisting of alkali-metals,alkaline earth metals, and ammonium, x is an integer selected from thegroup consisting of l and 2, and R, and R are selected from the groupconsisting of H, methyl, ethyl, propyl, butyl, amyl, and methyleneradicals which, when joined together, form with the nitrogen atom, apiperidine ring.

9. S-methyl-Z-vinyl pyridine stabilized against popcorn polymerformation, with about 0.01 to about 5.0 weight percent of potassiumdimethyl-dithiocarbamate.

l0. 5-methyl-2vinyl pyridine stabilized against popcorn polymerformation with about 0.01 to about 5.0 weight percent of sodiumdimethyl-dithiocarbamate.

11. A method of stabilizing 5-methyl-2-vinyl pyridine against popcornpolymer formation which comprises treating said pyridine with astabilizing amount of a compound selected from the group consisting ofcompounds according to the general formula R; S} was). R.

wherein M is a member selected from the group consisting ofalkali-metals, alkaline earth metals, and ammonium, x is an integerselected from the group consisting of 1 and 2, and R and R are selectedfrom the group consisting of H, methyl, ethyl, propyl, butyl, amyl, andmethylene radicals which, when joined together, form with the nitrogenatom, a piperidine ring.

12. A method of stabilizing a polymerizable vinylpyridine compoundagainst popcorn polymer formation with a stabilizing amount of acompound selected from the group consisting of compounds represented bythe general wherein M is a member selected from the group consisting ofalkali-metals, alkaline earth metals, and ammonium, x is an integerselected from the group consisting of 1 and 2, and R, and R are selectedfrom the group consist'rng of H, methyl, ethyl, propyl, butyl, amyl, andmethylene radicals which, when joined together, form with the nitrogenatom, a piperidine ring.

13. A polymerizable vinylpyridine compound stabilized against popcornpolymer formation by the addition of a stabilizing amount of a compoundselected from the group consisting of compounds represented by thegeneral foring of 1 and 2, and R and R are selected from the groupconsisting of H, methyl, ethyl, propyl, butyl, amyl, andmethylene'radicals which, when joined together, form with thenitrogenatom, a piperidine ring.

References Cited in the file of this patent UNITED STATES PATENTS2,386,272 Schulze Oct. 9, 1945 2,512,660 Mahan June 27, 1950 2,556,845Kaufiman June 12, 1951 2,592,625 Wagner Apr. 15, 1952 OTHER REFERENCESFrank et al.: J. Am. Chem. Soc., vol. 68, p. 908 (1946).

8. A POLYMERIZABLE HETEROCYCLIC BASE SELECTED FROM TEH GROUP CONSISTINGOF COMPOUND HAVING THE GENERAL FORMULAE